This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-097759, filed Mar. 29, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a magnetoresistance element and a magnetic memory.
2. Description of the Related Art
A magnetoresistance element includes a pair of ferromagnetic layers laminated one upon the other with a nonmagnetic layer interposed therebetween. The resistance value of the magnetoresistance element is changed in accordance with the direction of the magnetization of one ferromagnetic layer relative to the magnetization of the other ferromagnetic layer. The magnetoresistance element producing the particular magnetoresistance effect can be used in various fields, e.g., in a magnetic memory.
In a magnetic memory, one of the ferromagnetic layers acts as a pinned ferromagnetic layer that retains the direction of the magnetization thereof unchanged on applying a magnetic field, and the other ferromagnetic layer acts as a free ferromagnetic layer that is capable of changing the direction of the magnetization thereof on applying the magnetic field so as to store information. To be more specific, information is written in the magnetic memory by the resultant magnetic field generated when a current pulse passes through a word line and a bit line. As a result, the magnetization of the free ferromagnetic layer is changed between, for example, the parallel state and the antiparallel state, relative to the magnetization of the pinned ferromagnetic layer. In this fashion, binary information of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d is written in accordance with these two states. Also, when the written information is read out, an electric current is passed through the magnetoresistance element. Since the resistance value of the magnetoresistance element under one of the two states noted above, i.e., the parallel state, differs from that under the other state, i.e., the antiparallel state, it is possible to read out the written information by detecting the current flowing through the magnetoresistance element (the resistance value).
It should be noted that, in order to enhance the degree of integration of a magnetic memory, it is highly effective to make smaller the area of the magnetoresistance element. However, when an external magnetic field is not applied, or is too weak, a complex magnetic domain structure consisting of a plurality of domains is formed in the vicinity of the edge portion of the free ferromagnetic layer. If the area of the free ferromagnetic layer is made smaller, the proportion of the edge portion relative to the entire free ferromagnetic layer increases, with the result that, in, for example, an oblong free ferromagnetic layer, the direction of the magnetization in both edge portions in the longitudinal direction of the free ferromagnetic layer is rendered different from that in the central portion of the free ferromagnetic layer. In other words, a so-called xe2x80x9cedge domainxe2x80x9d is generated as described in, for example, xe2x80x9cJ. App. Phys.xe2x80x9d 81, 5471 (1997). In this case, the magnetization of the free ferromagnetic layer is lowered, which lowers the magnetoresistivity. Also, in this case, the change in the magnetic structure in reversing the magnetization is rendered complex. As a result, the possibility of noise generation is increased. In addition, the coercive force is increased, which increases the intensity of the magnetic field required for the switching (switching magnetic field).
Concerning the technology for suppressing the edge domain, it is known to the art that the shape of the free ferromagnetic layer is made asymmetric to the axis of easy magnetization thereof, particularly, the free ferromagnetic layer is made to have a shape of a parallelogram, as disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-273337. Where the free ferromagnetic layer is shaped as above, it is possible to make smaller the area of the edge domain so as to make it possible for the entire ferromagnetic layer to be formed of a substantially single magnetic domain.
Also, concerning the technology for preventing the change in the magnetic structure from being made complex in reversing the magnetization, it is known to the art that a structure to which a hard bias is applied is added to the both edge portions of the free ferromagnetic layer, as disclosed in, for example, U.S. Pat. No. 5,748,524 and Japanese Patent Disclosure No. 2000-100153.
Further, it is known to the art that a small portion projecting in a direction perpendicular to the axis of easy magnetization of the free ferromagnetic layer is formed in the free ferromagnetic layer so as to produce an H or I-shaped ferromagnetic layer instead of a simple quadrilateral, thereby stabilizing the edge domain and avoiding the formation of a complex magnetic domain, as disclosed in U.S. Pat. No. 6,205,053.
However, where the free ferromagnetic layer is made to have a shape of parallelogram, the coercive force is generally rendered excessively large.
Large coercive force stabilizes two stable states of magnetization and makes the stable states hard to be influenced by thermal agitation. Therefore, large coercive force is preferred in terms of stability of information stored. The increase in the coercive force, however, implies an increase of the switching magnetic field, since the magnitude of the coercive force provides a criterion of the magnitude of the switching magnetic field. To be more specific, it is thus necessary to flow a larger current into the write wiring for writing information in the magnetic memory, which brings about undesirable results, such as an increase in the power consumption and the shortening of the wiring life.
Also, where a structure to which a hard bias is applied is added to the both edge portions of the free ferromagnetic layer, the coercive force is increased, though it is certainly possible to control the behavior in terms of the change in the magnetic structure. In addition, in this technology, it is necessary to add a structure for stabilizing the edge domain and, thus, this technology is not adapted for the increase in the density required for a large capacity memory or the like.
Further, where the free ferromagnetic layer is H-shaped or I-shaped, it is necessary to enlarge the projecting portion in order to permit the projecting portion to produce a sufficient effect. In this case, the area occupied by the magnetoresistance element is increased, making it difficult to achieve the high degree of integration required for a large capacity memory.
According to a first aspect of the present invention, there is provided a magnetoresistance element, comprising a first pinned ferromagnetic layer that retains a magnetization direction thereof unchanged on applying a magnetic field, a free ferromagnetic layer that faces the first pinned ferromagnetic layer and is capable of changing a magnetization direction thereof on applying the magnetic field, and a first nonmagnetic layer intervening between the first pinned ferromagnetic layer and the free ferromagnetic layer, wherein a shape of the free ferromagnetic layer that is viewed perpendicularly to a main surface thereof includes a first portion with a parallelogrammic contour and a pair of second portions that protrude from a pair of opposite corners of the first portion respectively in a main direction parallel to a pair of opposite sides of the first portion, the shape of the free ferromagnetic layer is asymmetric with respect to a line that passes through a center of the first portion and is parallel to the main direction, and an axis of easy magnetization of the free ferromagnetic layer falls within a range defined by an acute angle that a first direction makes with a second direction, the first direction being substantially parallel to the main direction and the second direction being substantially parallel to the longest line segment that joins contours of the second portions.
According to a second aspect of the present invention, there is provided a magnetoresistance element, comprising a first pinned ferromagnetic layer that retains a magnetization direction thereof unchanged on applying a magnetic field, a free ferromagnetic layer that faces the first pinned ferromagnetic layer and is capable of changing a magnetization direction thereof on applying the magnetic field, and a first nonmagnetic layer intervening between the first pinned ferromagnetic layer and the free ferromagnetic layer, wherein a shape of the free ferromagnetic layer that is viewed perpendicularly to a main surface thereof includes a first portion with a quadrilateral contour whose first opposite sides are parallel to each other and whose second opposite sides are parallel to each other, and a pair of second portions that extend from a pair of opposite corner parts of the first portion in a main direction parallel to the second opposite sides respectively and whose maximum widths in a direction parallel to the first opposite sides are narrower than lengths of the first opposite sides, the shape is asymmetric with respect to a line that passes through a center of the first portion and is parallel to the second opposite sides, and an axis of easy magnetization of the free ferromagnetic layer falls within a range defined by an acute angle that a first direction makes with a second direction, the first direction being substantially parallel to the main direction and the second direction being substantially parallel to the longest line segment that joins contours of the second portions.
According to a third aspect of the present invention, there is provided a magnetoresistance element, comprising a first pinned ferromagnetic layer that retains a magnetization direction thereof unchanged on applying a magnetic field, a free ferromagnetic layer that faces the first pinned ferromagnetic layer and is capable of changing a magnetization direction thereof on applying the magnetic field, and a first nonmagnetic layer intervening between the first pinned ferromagnetic layer and the free ferromagnetic layer, wherein a shape of the free ferromagnetic layer that is viewed perpendicularly to a main surface thereof includes a first portion with a parallelogrammic contour and a pair of second portions that protrude from a pair of opposite corners of the first portion respectively in a main direction parallel to a pair of opposite sides of the first portion, the shape of the free ferromagnetic layer is asymmetric with respect to a line that passes through a center of the first portion and is parallel to the main direction, and a direction of the magnetization of the first pinned ferromagnetic layer falls within a range defined by an acute angle that a first direction makes with a second direction, the first direction being substantially parallel to the main direction and the second direction being substantially parallel to the longest line segment that joins contours of the second portions.
According to a fourth aspect of the present invention, there is provided a magnetoresistance element, comprising a first pinned ferromagnetic layer that retains a magnetization direction thereof unchanged on applying a magnetic field, a free ferromagnetic layer that faces the first pinned ferromagnetic layer and is capable of changing a magnetization direction thereof on applying the magnetic field, and a first nonmagnetic layer intervening between the first pinned ferromagnetic layer and the free ferromagnetic layer, wherein a shape of the free ferromagnetic layer that is viewed perpendicularly to a main surface thereof includes a first portion with a quadrilateral contour whose first opposite sides are parallel to each other and whose second opposite sides are parallel to each other, and a pair of second portions that extend from a pair of opposite corner parts of the first portion in main a direction parallel to the second opposite sides respectively and whose maximum widths in a direction parallel to the first opposite sides are narrower than lengths of the first opposite sides, the shape is asymmetric with respect to a line that passes through a center of the first portion and is parallel to the second opposite sides, and a direction of the magnetization of the first pinned ferromagnetic layer falls within a range defined by an acute angle that a first direction makes with a second direction, the first direction being substantially parallel to the main direction and the second direction being substantially parallel to the longest line segment that joins contours of the second portions.
According to a fifth aspect of the present invention, there is provided a magnetic memory, comprising a word line, a bit line intersecting the word line, and a memory cell positioned at or near an intersection portion of the word and bit lines and including the element according to one of first to fourth aspects of the present invention.
It is noted that xe2x80x9can axis (or direction). . . falls within a range defined by an acute angle that a first direction makes with a second directionxe2x80x9d corresponds to the configuration that the axis (or direction) is parallel to one of the first and second directions or the configuration that the axis (or direction) intersects each of the first and second directions at an angle narrower than the acute angle noted above.
In the first to fifth aspects of the present invention, it is possible for the first nonmagnetic layer to be a nonmagnetic metal layer or an insulating layer.
Also, it is possible for a direction of the magnetization of the first pinned ferromagnetic layer to fall within the range defined by the acute angle.
Further, it is possible for the magnetoresistance element to further comprise a second pinned ferromagnetic layer that retains a magnetization direction thereof unchanged on applying the magnetic field, and a second nonmagnetic layer intervening between the free ferromagnetic layer and the second pinned ferromagnetic layer. In this case, it is possible for the first and second pinned ferromagnetic layer to be a nonmagnetic metal layer or an insulating layer.
As described above, it is possible for the magnetoresistance element to be an element exhibiting a giant magnetoresistance effect or a ferromagnetic tunnel junction element such as a ferromagnetic single tunnel junction element having a ferromagnetic single tunnel junction formed therein or a ferromagnetic double tunnel junction element having a ferromagnetic double tunnel junction formed therein.
In the first to fifth aspects of the present invention, it is possible for the first portion to be square or rectangle in shape.
Also, it is possible for the second portions to be rotation symmetrical with respect to a 2-fold axis that passes through the center of the first portion and is perpendicular to the main surface of the free ferromagnetic layer. Further, it is possible for each of the two second portions to be triangle, semi-circle, square or rectangle in shape.
Still further, it is possible for the shape of the free ferromagnetic layer to consists of the first and second portions.
Incidentally, the expression xe2x80x9csubstantially parallelxe2x80x9d referred to above implies that the deviation from the parallel state is within several degrees.